George L. Streetcr 105 



striicticm. The slia])^ ol' tlie civst at this time is' ri'iircsi'iiled in Figs. 1, 

 2, 3, and Plate I. In the iigures the presence of dcvel()|)ing nerve fibres 

 are diagranimatically shown among the cells of the crest. Small bundles 

 of these fibres spring at irregular intervals from the lateral angle of the 

 neural tube and enter the crest. In the caudal two-thirds these fibres 

 join to form a definite strand, wliich is the primitive trunk of the acces- 

 sory nerve. This trunk reaches down into the region of the spinal gang- 

 lion crest to the level of the third or fourth cervical segment. The fibres 

 do not, however, enter this crest, but run along median to it as far as the 

 first cervical, when they enter the vagus crest as though into a sleeve. 

 In regard to the first cervical a variability is shown, the trunk may run 

 median, lateral, or through it. Forward, in a line with these fibres of the 

 accessory trunk, are found a few others forming small scattered bundles 

 at the head of the crest. That the fibres which are present at this stage 

 are motor may be inferred from three facts : firstly, they spring from the 

 lateral horn region of the neural tube; secondly, there is at this time no 

 apparent fibroblast development in tlie cells of the ganglion crest; and 

 thirdly, some of them can be followed in their further development until 

 they become known motor elements, as in the case of the main trunk of 

 the accessorius, and as in the spinal cord where the ventral roots at this 

 time are well laid out, though there is as yet no trace of dorsal roots. 



Ventral to the head of the vagus crest, and partially separated from it 

 by a looser zone of cells, is found a second ganglionic mass, the primitive 

 ganglion nodosum, the relation between Avhich and the crest repeats the 

 condition found betw'een the ganglion of the root and the ganglion of the 

 trunk of the glosso-pharyngeus, evidence of independence being here 

 equally strong. As was there pointed out, the ganglion nodosum is 

 closely associated with the development of the more caudal branchial 

 arches. The patch of thickened epidermis over the ganglion, as in Fig. 6, 

 represents an epibranchial sense organ. A complete segmentation of 

 the ganglion nodosum, which might be expected in considering this anlage 

 as the morphological equivalent of a series of gill cleft ganglia, is not 

 found, though the cells show at first a loose irregular grouping, which 

 represents perhaps a branchio-meric tendency. The laryngeal branch of 

 this ganglion is present in 7.0 mm. embryos, and forms the principal 

 nerve to the fourth branchial arch. The main vagus trunk is differen- 

 tiated at about the same time and is seen sprouting out from the distal 

 end of the ganglion. 



At the end of the first month (Figs. 7, 8, and 9) the cellular column 

 between the ganglion nodosum and the ganglion crest is converted into 

 a fibrous trunk. At tliis time the ganglion crest, besides an increase in 



